Chikungunya virus (CHIKV) is a re-emerging mosquito-borne alphavirus that cycles directly between humans and mosquitoes and causes a debilitating febrile illness in humans on a global scale. Autochthonous transmission in the New World now has been described, with outbreaks in at least 19 countries in the Americas and over a quarter million cases in just 6 months. CHIKV disease cases have been reported to ArboNET in travelers returning to the US from the Caribbean in 29 states, as of July 1. The potential for epidemics in North and South America is high due to the ubiquitous distribution of one of its primary vectors, Aedes albopictus. Despite the possibility for infecting and causing disease in millions, specific treatments or vaccines for CHIKV are not available. A primary goal of this project is to define the molecular, genetic, immunologic, and structural characteristics of ultra-potent neutralizing human mAbs with broad activity against all genotypes of CHIKV. Additional goals include defining the mechanistic correlates of protection by these ultra-potent neutralizing mAbs. In these studies, we will elucidate how antiviral Abs with exceptional inhibitory activity exert their action in cell culture and in vivo. The approach will include high efficiency isolationof human mAbs, coupled with innovative antibody gene repertoire studies based on nextgen sequencing. Several hypotheses will be tested, including the concept that ultra-potent neutralizing activity results from features of both the antibodies (extensive mutations due to persistent CHIKV infection) and the antigen (binding to quaternary epitopes on multiple adjacent envelope proteins and blocking structural transitions critical for virus entry or release). Althoug our focus is to understand how and why ultra-potent human mAbs inhibit CHIKV, the studies likely will be relevant to general principles of antibody neutralization of many different viruses. Beyond defining the molecular and structural basis of Ab neutralization of CHIKV, these studies will generate a group of fully human mAbs that can prevent and treat CHIKV infection and persistence in mice, which could be developed in the future as a possible combination immunotherapeutic for humans. Studies in this project, while targeted against CHIKV, likely will inform future Ab-based and/or vaccine efforts against other alphaviruses that cause human disease. We have assembled a unique group of investigators, including a human Ab expert, a molecular virologist with experience in Ab-virus interactions, and two accomplished structural biologists with specific expertise in alphaviruses, including CHIKV, to pursue these studies.